1. Field of the Invention
The present invention relates to the field of magnets. In particular, the present invention relates to a combination of magnets, either electromagnets or permanent magnets, for generating a very uniform external magnetic field in a plane disposed proximate to the combination of magnets.
2. Description of the Related Art
Magnets, both permanent magnets and electromagnets, are used in applications that exploit the magnets' unique capability to provide a force, or perform work of some kind, while avoiding contact with the object on which the force is exerted, or work performed. A magnetic structure performs work through the generation of a magnetic field external to itself. Typically, the object upon which the magnet operates is disposed proximate the magnet such that the external magnetic field generated by the magnet produces a desired effect on the object. The greater the strength of the magnetic field generated by the magnet, the greater the magnet's ability to perform work. To that end, research has focused on techniques for improving the efficiency of the magnetic circuit formed by a magnet, whether permanent magnet or electromagnet, to maximize the strength of the external magnetic field generated by the magnets while minimizing the volume of magnetic material (in a permanent magnet) or the size of the core/coils (in an electromagnet).
Additionally, in certain applications, such as ion beam deposition, wherein atoms are stripped from a magnetic target for depositing on a substrate, magnets are used to improve the deposition process, for example, by increasing target utilization/yield and deposition rate. In such applications, not only is increasing the flux density, B, of the external magnetic field generated by the magnets important, but so is controlling the direction and uniformity of the flux lines within the external magnetic field. Likewise, controlling uniformity in flux density in the external magnetic field is important. Thus, the prior art focuses on controlling these aspects of the external magnetic field generated by a permanent magnet structure or electromagnet to, for example, increase substrate yield and minimize magnet size and equipment cost.
U.S. Pat. No. 5,630,916, issued to Gerrish et al. discuss the art of manufacturing magnetic recording heads, in which thin films of magnetic material having a particular magnetic field orientation are deposited by a sputtering apparatus on a substrate. The process involves the placement of a magnet near the substrate to expose the substrate to a magnetic field having uniform magnitude field lines extending in a uniform direction, that is, a uniform magnetic field, thereby causing the magnetic material to be deposited on the substrate in a predetermined magnetic orientation. In this manner, the magnetic domains in the magnetic film being formed on the surface of the substrate can be oriented in the same direction. However, Gerrish et al. point out that, in the prior art, only a small portion of the magnetic field generated by the magnet has the necessary uniformity, thus limiting the area of the substrate over which the target material is deposited in the desired magnetic orientation. Yet commercial demand calls for the use of a larger substrate, coated with a magnetic film having more accurately aligned magnetic orientation, over a larger portion of the substrate, to improve substrate yield.
FIG. 1 illustrates a prior art dipole permanent magnet plate 100 made of, for example, annealed low carbon steel, or Alnico. A magnetized state in plate 100 provides for a north pole 108 along one edge 107 and a south pole 110 along an opposing edge 109 such that an external magnetic field is generated by leakage flux over the surfaces of the plate between the north and south pole. The magnetic lines of force, i.e., the flux lines, are illustrated with respect to coordinates x, y and z. The flux lines all flow from the north pole to south pole, and thus repel one another, so that the flux lines tend to diverge as they move away from a pole, rather than converge or remain parallel. The flux lines, then, tend to follow parallel paths 104 along a central axis 101 in the direction of the coordinate x, but progressively diverge in the direction of the coordinate y as one moves along the plate from the central axis 101 to the edges 103 or 105 orthogonal to the north and south poles at edges 107 and 109 respectively. The uniformity of the magnetic field in the x coordinate direction in a substantially parallel plane above or below the plate varies, then, from being substantially uniform over the center of the plate and along the center axis, to being less so as one moves along the plane in the y coordinate direction. That is, the uniformity in the direction of the lines of flux, as well as the uniformity of flux density, in a substantially parallel plane above or below the plate decreases as one moves from the center of the plate, particularly along the plane in the y coordinate direction. This is best illustrated with reference to FIG. 3, which provides a top view of a prior art plate 300. Flux lines at or near 304 in the center of the plate flow substantially in the x coordinate direction, from north pole 308 to south pole 310, while flux lines 302 at or near the edges of the plate include a significant y coordinate component to their direction.
FIG. 2 illustrates a plate-shaped electromagnet 200, having a core made of, for example, cold rolled steel. The core is wrapped in a coil of electrically conductive wire 212 that is connected to a power supply (not shown). When power is supplied by the power supply to the coil, electrical current flowing through the coil induces a magnetic field normal to the direction of electrical current flow, identical to the magnetic field generated by the dipole permanent magnet plate 100 in FIG. 1. Thus, the electromagnet of FIG. 2, just as the permanent magnet of FIG. 1, provides for a north pole 208 along one edge 207 and a south pole 210 along an opposing edge 209 such that an external magnetic field is generated by magnetic flux lines passing over the surfaces of the plate between the north and south pole. The flux lines progressively diverge in the direction of the coordinate y as one moves along the plate shaped electromagnet from the central axis 201 to the edges 203 or 205 orthogonal to the north and south poles. The uniformity in the direction of the lines of flux, in a substantially parallel plane above or below the plate decreases as one moves from the center of the plate, particularly along the plane in the y coordinate direction.
The size of an object disposed in the parallel plane proximate the plates of FIG. 1 or 2 and over which it is desired to induce a substantially uniform magnetic field is, therefore, restricted to an area somewhat less than that defined by the plates. What is needed is an arrangement or combination of magnets that expands the area of substantial uniformity of the magnetic field in the plane proximate the plate such that the object disposed in the plane, e.g., a wafer, is not so restricted in size relative to the size of the plate.
Gerrish et al. disclose obtaining a substantially uniform magnetic field in a plane proximate a plate shaped electromagnet by varying either the density of the windings or the amount of current flowing in the windings wrapped around the electromagnetic plate, so that increased electrical current flows through the windings near the ends of the plate-shaped core to compensate for changes in the strength of the magnetic field at or near the ends. This approach, however, is limited to electromagnetic arrangements, and may require variable and/or multiple power supplies. What is needed is a magnet arrangement for providing a desired uniform external magnetic field that may incorporate permanent and/or electromagnets, and which does not require multiple, sophisticated, and expensive power supplies.
Additionally, the Gerrish et al. approach produces uniform direction of the magnetic field in the area of a plane proximate the electromagnetic plate having a skew of at least .+-.one degree or more from the so-called "easy" axis. However, present applications for such magnet arrangements require an angular dispersion of not more than .+-.0.5 degrees from the desired direction of the flux lines comprising the external magnetic field.
The prior art use of magnetically permeable field shapers positioned next to the plate shaped magnet provide for an increase in the size of the area in the plane proximate to the plate shaped magnet over which a substantially uniform magnetic field may be achieved. However, such arrangements still fail to provide angular dispersion of not more than .+-.0.5 degrees from the desired direction of the flux lines comprising the external magnetic field.
Thus, what is needed is a novel arrangement of magnets that generates an external magnetic field having a highly uniform magnetic field orientation over a wider area of a plane disposed proximate to the arrangement of magnets than heretofore provided by prior art arrangements.